The present invention relates in general to cutting instruments, and in particular to a micromachined cutting blade formed of monocrystalline silicon having a {211} crystalline orientation, and to a method for manufacture thereof.
Laser in-situ keratomelleusis (LASIK) is a procedure that is widely used for laser eye surgery to correct refractive errors producing nearsightedness, farsightedness or astigmatism. The use of the LASIK procedure requires a mechanical keratome to cut a thin circular flap across the cornea of a patient""s eye so that an eximer laser can then be used to remove a calibrated amount of underlying tissue from the cornea to achieve a desired refractive change. The mechanical keratome as presently used comprises a stainless steel knife blade which is rapidly oscillated during use.
Metal cutting blades can have ragged or uneven cutting edges, and can form burrs extending outward from the cutting edge. Metal cutting blades can also dull during surgical use. These defects, if present on the blade""s metal cutting edge, can produce incisions that are ragged and uneven and that extend beyond a desired or critical depth. This can be harmful in reducing the precision of the surgical procedure, in increasing the time required for healing after surgery, and in affecting the patient""s vision correction as a result of the surgery. To limit these harmful effects, a careful microscopic inspection of the blade""s cutting edge during manufacture and before use must be performed which increases the cost of the blades and results in some blades being rejected as unsuitable for surgery. What is needed is a knife blade for keratomy that has a high degree of sharpness, and which can be reliably produced at low cost. Also needed is a knife blade which maintains its sharpness during use, and is not subject to the formation of burrs.
The present invention provides a solution to this problem by providing a cutting blade (i.e. a knife blade) formed of monocrystalline silicon that can be precisely sharpened by anisotropic etching and which, in the absence of any applied coatings, is incapable of developing burrs because of its crystalline nature.
The use of monocrystalline silicon with a {100} substrate orientation for forming a knife blade is disclosed in U.S. Pat. No. 5,579,583 to Mehregany et al. Mehregany""s requirement for a {100}-oriented substrate produces a blade having a cutting angle determined by the intersection of two crystalline planes, with the cutting angle being crystallography fixed at a relatively large blade angle of either 54.7xc2x0 or 109.4xc2x0. These relatively large blade angles are disadvantageous for use in eye surgery since the large blade angles would effectively reduce the sharpness of the blade and would also make it difficult for the delicate cornea flap to easily slide across the blade during cutting.
The use of monocrystalline silicon for forming various types of knife blades is also disclosed in U.S. Pat. No. 5,842,387 to Marcus et al; U.S. Pat. No. 5,928,161 to Krulevitch et al; and U.S. Pat. No. 5,980,518 to Carr et al. None of these references disclose the use of a {211} substrate orientation for forming a knife blade as used according to the present invention. Additionally, none of these references disclose a cutting edge formed in monocrystalline silicon by a pair of substantially planar cutting-edge surfaces aligned along crystalline planes of silicon and intersecting at an angle of less than 30 degrees.
An advantage of the present invention is that a cutting blade can be fabricated that is substantially free from any burrs or ragged cutting edges.
Another advantage of the present invention is that a cutting blade can be formed with a shallow cutting-edge angle of less than 30 degrees, and preferably less than 20 degrees.
A further advantage of the present invention is that the cutting edge of the blade can be coated with a deposited material such as silicon nitride, titanium nitride, tungsten, amorphous diamond or parylene for improved wear resistance, reduced friction or biocompatibility.
Yet another advantage of the present invention is that a plurality of cutting blades can be formed on a single silicon substrate (i.e. a wafer) in a batch fabrication process and then be individually separated.
Still another advantage of the present invention is that single-edged and double-edged cutting blades can be formed according to the present invention.
These and other advantages of the method of the present invention will become evident to those skilled in the art.
The present invention relates to a micromachined cutting blade that comprises an elongate body of monocrystalline silicon having a pair of substantially parallel major body surfaces, with each major body surface being aligned substantially coplanar with a {211} crystalline plane of silicon, and a substantially planar cutting edge formed in the monocrystalline silicon body at an angle to one of the major body surfaces and oriented along the length of the body. The cutting-edge angle is preferably 19.5 degrees and corresponds to the intersection of a {211} crystalline plane of silicon with a {111} crystalline plane of silicon. The cutting edge is formed by anisotropically etching the monocrystalline silicon body, with the etching terminating at a {111} crystalline plane of silicon. In some embodiments of the present invention, the cutting edge of the blade can be hardened for increased wear resistance by forming a coating of a hard material over at least a part of the cutting edge. The coating can comprise silicon nitride, titanium nitride, tungsten, or amorphous diamond. Alternately, a conformal parylene coating can be formed over a portion or the entirety of the cutting blade. The cutting blade can also include a handle connected to opposite ends of the crystalline silicon body to support the body in tension, thereby keeping the cutting edge planar. Such a handle can be, for example, U-shaped.
The present invention further relates to a micromachined cutting blade that comprises an elongate body of monocrystalline silicon having a pair of substantially parallel major body surfaces, and at least one cutting edge formed in the monocrystalline silicon body, with each cutting edge further comprising a pair of cutting-edge surfaces aligned along crystalline planes of silicon and intersecting at an angle of generally less than 30 degrees, and preferably less than 20 degrees. One of the surfaces of each cutting edge is aligned substantially coplanar with one of the body surfaces which, in turn, is substantially coplanar with a {211} crystalline plane of silicon. The other surface of each cutting edge is aligned substantially along a {111} crystalline plane of silicon. A coating of a hard material (e.g. silicon nitride, titanium nitride, tungsten, or amorphous diamond) can be provided to cover at least a part of one cutting edge of the blade to increase its wear resistance. Alternately, a conformal parylene coating can be formed over at least a portion of the cutting blade.
The present invention also relates to a method for forming a micromachined cutting blade, comprising steps for providing a monocrystalline silicon body having a pair of substantially parallel major body surfaces, with each major body surface being aligned substantially along a {211} crystalline plane of silicon; and forming at least one cutting edge in the monocrystalline silicon body by forming an etch mask over each body surface, with the etch mask formed over at least one of the body surfaces having an elongate opening therethrough to expose a portion of the body surface wherein the cutting edge is to be formed; anisotropic etching the exposed portion of the body surface through the opening in the etch mask down to the opposite body surface; and removing each etch mask. Each cutting edge is aligned substantially along a {111} crystalline plane of silicon. This can be done by using an anisotropic wet etchant such as potassium hydroxide (KOH), tetramethyl ammonium hydroxide (TMAH) or ethylenediamine pyrocatechol (EDP).
Each cutting edge can also be hardened by depositing a coating of a hard material (e.g. silicon nitride, titanium nitride, tungsten or amorphous diamond) over at least a part of the cutting edge. This coating can be deposited by a conventional vapor deposition process. Alternatively, a conformal coating of parylene can be deposited over at least a portion of each cutting blade.
Finally, a handle can be attached to the monocrystalline silicon body to hold the monocrystalline silicon body and each cutting edge in tension. Such a handle can be, for example, U-shaped.
Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.